As conventionally well-known, in order to efficiently generate electricity in a proton-exchange membrane fuel cell, it is desirable to keep the electrolyte membrane moderately wet and avoid an excessive or insufficient moisture content in the fuel cell. There is a known technique of controlling a moisture content in a cell plane of a fuel cell, as described in Patent Document 1 (Japanese Patent Laid-Open No. 2004-335444), for example. Patent Document 1 discloses adjusting one of pressure drop characteristics due to pressure, humidity, temperature, flow rate, and flow channel geometry of reactant gas, which is a general term for oxidation gas represented by air and fuel gas represented by hydrogen gas, to control the distribution of moisture content present in the cell plane as droplet or water vapor.